The invention has its origin in problems that are associated with milling inserts, more precisely such replaceable milling inserts that are included in tools for so-called hobbing or gear hobbing. Gear hobbing is a relatively new milling method by means of which slots can be milled out of the envelope surface of metallic workpieces having a rotationally symmetrical shape with the ultimate purpose of forming ridges, e.g. in the form of cogs, spline bars or the like, which generally run parallel to the centre axis of the surface of revolution. Briefly, this special milling method means that the workpiece is set in rotation at a moderate rotational speed, at the same time as a high-speed rotating milling tool, which is equipped with milling inserts located in screw formations in relation to the rotation axis of the tool, is given a slow, axial feeding motion during which the individual milling insert first enters the workpiece while removing a relatively small chip, and then leaves (“rolls off”) the workpiece. In a later moment, when the workpiece has rotated one revolution, a milling insert, which has been fed forward axially a short distance, will once again enter the initiated slot and remove more material from the workpiece. This is repeated until the recessed slot has obtained the desired length. In comparison with traditional gear milling, gear hobbing affords the advantage that the machining can be carried out faster and more cost-effective.
Modern milling inserts for gear hobbing are so-called full profile inserts, i.e., milling inserts that in the moment of engagement remove a chip along two converging main edges as well as along an end or nose edge in one and the same pass.
Within all forms of cutting or chip removing machining of metal blanks, the design of the cutting edges of the cutting inserts has a large bearing on the performance of the tool, above all in respect of the dimensional accuracy and surface finish of the manufactured components. In this respect, hob cutters are no exception. On the contrary, the dimensional accuracy of the flank surfaces that delimit the slots between adjacent ridges of the workpiece, e.g. the gashes between two adjacent cogs, has a large bearing on the function of the component. In many cases, no finishing is required if the generated flank surfaces already during the milling obtains a good dimensional accuracy and surface finish. Even if finishing would be called for in individual cases, the waste of time and the cost therefor is, however, decreased, if the critical surfaces of the milled component already initially have a good dimensional accuracy.
In this connection, it should be pointed out that the basic body of the milling tool usually is manufactured from steel, while the replaceable milling inserts are manufactured from a material that is harder and more wear-resistant than steel, e.g. cemented carbide or the like. This usually takes place in facilities that are separated from each other. When the milling inserts are manufactured from cemented carbide, the outcome in the manufacture may vary from milling insert to milling insert. Among other things, the micro geometry of the cutting edges may vary. For this reason, directly pressed milling inserts, i.e., milling inserts that are not ground, obtain their final shape already in connection with pressing and sintering, a mediocre dimensional accuracy in the context, above all in respect of the exact spatial location of the cutting edge line. The requirements of dimensional accuracy of the cutting edges are at times extreme and may be about 0.01 mm or less.
Like other replaceable milling inserts, milling inserts have a limited service life (often within the range of 5-10 min), and therefore the same are mass-produced in large series. As a consequence of the limited service life, frequent insert replacements are required to maintain a good manufacturing precision. This means that the cost of the milling inserts has a vital importance to the user and his/her possibilities to keep down the manufacturing cost. Of the total cost of a high-performance milling insert, the cost of grinding is a considerable part.